1. Field
The present disclosure relates to a liquefied natural gas tank.
2. Discussion of the Related Technology
Generally, natural Gas (NG) is turned into a liquid (also called liquefied natural gas or LNG) in a liquefaction plant, transported over a long distance by an LNG carrier, and re-gasified by passing a floating storage and re-gasification unit (FSRU) or an unloading terminal on land to be supplied to consumers.
In case LNG is transported by an LNG re-gasification vessel (LNG-RV), LNG is re-gasified in the LNG-RV itself, not passing a FSRU or an unloading terminal on land, and then supplied directly to consumers.
As liquefaction of natural gas occurs at a cryogenic temperature of approximately −163° C. at ambient pressure, LNG is likely to be vaporized even when the temperature of the LNG is slightly higher than −163° C. at ambient pressure. Although an LNG carrier has a thermally insulated LNG storage tank, as heat is continually transferred from the outside to the LNG in the LNG storage tank, the LNG is continually vaporized and boil-off gas is generated in the LNG storage tank during the transportation of LNG. If boil-off gas is generated in an LNG storage tank as described above, the pressure of the LNG storage tank is increased and becomes dangerous.
Generally, to maintain a constant pressure within the LNG storage tank for an LNG carrier, the boil-off gas generated in the LNG storage tank is consumed as a fuel for propulsion of the LNG carrier. That is to say, LNG carriers for transporting LNG basically maintain the temperature of the LNG in the LNG storage tank at approximately −163° C. at ambient pressure by discharging the boil-off gas to the outside of the tank.
For example, a steam turbine propulsion system driven by the steam generated in a boiler by burning the boil-off gas generated in an LNG storage tank has a problem of low propulsion efficiency. Also, a dual fuel diesel electric propulsion system, which uses the boil-off gas generated in an LNG storage tank as a fuel for a diesel engine after compressing the boil-off gas, has higher propulsion efficiency than the steam turbine propulsion system. But it has difficulty in maintenance due to complicated integration of a medium-speed diesel engine and an electric propulsion unit in the system. In addition, this system employs a gas compression method which requires higher installation and operational costs than a liquid compression method. Further, such method using boil-off gas as a fuel for propulsion fails to achieve the efficiency similar to or higher than that of a two-stroke slow-speed diesel engine, which is used in ordinary ships.
There is also a method of re-liquefying the boil-off gas generated in an LNG storage tank and returning the re-liquefied boil-off gas to the LNG storage tank. However, this method of re-liquefying the boil-off gas has a problem of installing a complicated boil-off gas re-liquefaction plant in the LNG carrier.
Furthermore, when the amount of boil-off gas generated in an LNG storage tank exceeds the capacity of a propulsion system or a boil-off gas re-liquefaction plant, the excessive boil-off gas needs to be burnt by a gas combustion unit or gas burner. Consequently, such method has a problem of needing an auxiliary unit such as a gas combustion unit for treating excessive boil-off gas.
For example, as illustrated in FIG. 4, in a case of an exemplary LNG carrier which basically maintains an almost constant pressure in an LNG storage tank, the LNG storage tank is somewhat hot for the first time (for 3 to 5 days after LNG is loaded therein). Consequently, as indicated by the solid line at the upper part of the diagram, a considerably large amount of excessive boil-off gas, compared with the amount of natural boil-off gas (NBOG), is generated during the transportation of LNG, and this excessive boil-off gas exceeds the amount of fuel consumed by a boiler or duel fuel diesel electric propulsion system. Accordingly, the amount of boil-off gas corresponding to the area indicated by oblique lines which shows a difference from the dotted line at a lower part of the diagram illustrating the amount of boil-off gas used in a boiler or engine may need to be burnt by a gas combustion unit (GCU). In addition, when an LNG carrier passes a canal (e.g. between 5 and 6 days in FIG. 4), as boil-off gas cannot not consumed in a boiler or engine (when the LNG carrier is waiting to enter a canal), or a small mount of boil-off gas is consumed (when the LNG carrier is passing a canal), the excessive boil-off gas which has not been consumed for propulsion of an engine needs be burnt. Further, even when the LNG carrier with LNG loaded therein is waiting to enter port or entering port, none or a small amount of boil-off gas is consumed, and consequently the excessive boil-off gas needs be burnt.
In a case of an LNG carrier having a capacity of 150,000 m3, boil-off gas burnt as described above amounts to 1500 to 2000 tons per year, which cost about 700,000 USD, and the burning of boil-off gas raises a problem of environmental pollution.
Korean Patent Laid-Open Publication Nos. KR 10-2001-0014021, KR 10-2001-0014033, KR 10-2001-0083920, KR 10-2001-0082235, and KR 10-2004-0015294 disclose techniques of suppressing the generation of boil-off gas in an LNG storage tank by maintaining the pressure of the boil-off gas in the LNG storage tank at a high pressure of approximately 200 bar (gauge pressure) without installing a thermal insulation wall in the LNG storage tank, unlike the low-pressure tank as described above. However, this LNG storage tank have a significantly high thickness to store boil-off gas having a high pressure of approximately 200 bar, and consequently it has problems of increasing manufacturing costs and requiring additional components such as a high-pressure compressor, to maintain the pressure of boil-off gas at approximately 200 bar. There is also a technique of a pressure tank, which is different from the above-mentioned technique. As highly volatile liquid is stored in a super high-pressure tank, for example, at a pressure higher that 200 bar and at the room temperature, this super high-pressure tank does not have a problem of treating boil-off gas, but has other problems that the tank should be small, and that the manufacturing costs are increased.
As stated above, an LNG storage tank for an LNG carrier, which maintains the pressure of cryogenic liquid constant near ambient pressure during the transportation of the LNG and allows generation of boil-off gas, has a problem of consuming a large amount of boil-off gas or installing an additional re-liquefaction apparatus. In addition, a method of transporting LNG using a tank, such as a high pressure tank, which withstands a high pressure at a high temperature, unlike a tank which transports said cryogenic liquid at a low atmospheric pressure, does not need to treat boil-off gas, but has a limitation on the size of the tank and requires high manufacturing costs.
The discussion in this section is to provide general background information and does not constitute an admission of prior art.